From electrospray ionization to cold-spray ionization: How to evaluate the cooling effect on the gaseous ions?

Two methods of survival yields (SY) measurement treatment of thermometer ions whose fragmentation is activated by in-source collision induced dissociation have been investigated for evaluating the mean internal (int >) and thermal (therm >) energies of gaseous ions produced by electrospray ionization and cold-spray ionization (CSI). One of the methods is based on the use of the internal energy distributions (P (E_int )) as sigmoid derivatives connecting the experimental survival yields of different substituted benzylpyridinium cations. The int > values are therefore converted in a thermal-like parameter called vibrational temperature (T_vib ) then obtained at each value of the voltage of the desolvation area. The second method is based on the modelling of ion behavior by the MassKinetics software where the value of the characteristic temperature parameter (T_char ) is used for fitting theoretical survival yields (SY_theo ) with experimental data (SY_exp ) calculated at several activation energy. A linear correlation is evidenced between the values of internal or thermal energy and the voltage of the orifice 1 at the origin of the ion activation in the desolvation area. The extrapolation at zero voltage of the thermal-like parameters (T_vib and T_char ) indicates that, in agreement with the literature data, the ions are relatively hot in ESI (~650 K). But the use of a CSI source lowers this temperature down to ~300 K. In addition, with cold-spray ionization, this cooling effect is more important when methanol is used instead of acetonitrile although these two solvents have no influence on the gaseous ion temperature in electrospray ionization.